In a digital amplifying apparatus, a digital amplifier including a positive-phase amplifier and an inverting amplifier. An output terminal is connected to a speaker through a so-called Balanced Transformerless connection (hereinafter, “BTL connection”). Waveforms having inverse phases to each other are output from the positive-phase amplifier and the inverting amplifier, respectively, due to class-D operations at each amplifier. Such a digital amplifying apparatus can facilitate an improvement of output voltage over that of an apparatus equipped with an ordinary amplifier.
FIG. 1 is a schematic block diagram of a schematic configuration of a digital amplifying apparatus including a digital amplifier executing a class-D operation (hereinafter, “class-D amplifier”). As shown in FIG. 1, the digital amplifying apparatus is configured by a class-D amplifying unit 101 and an amplifier output processing unit 102 that processes output from the class-D amplifying unit 101. The class-D amplifying unit 101 includes a positive-phase amplifier 101a and an inverting amplifier 101b. The amplifier output processing unit 102 includes low-pass filters (hereinafter, “LPF”) 102a and 102b, low-pass filter 102a connected to output terminals of the positive-phase amplifier 101a, and low-pass filter 102b connected to output terminals of the inverting amplifier 101b. 
The digital amplifying apparatus having the above configuration is configured by connecting the output terminals of the positive-phase amplifier 101a and the inverting amplifier 101b to a speaker 103 through the LPFs 102a and 102b and, thereby, the positive-phase amplifier 101a and the inverting amplifier 101b function for the speaker 103 as one BTL-connected amplifier.
In a digital amplifying apparatus, an input digital audio signal is converted by PWM (Pulse Width Modulation) into a pair of PWM signals, each having a phase inverse of the other. Of the PWM signals, a positive PWM signal is provided to the positive-phase amplifier 101a and a negative PWM signal is provided to the inverting amplifier 101b. 
The positive-phase amplifier 101a and the inverting amplifier 101b realize a class-D operation. This operation causes the positive and the negative PWM signals to switch predetermined DC voltage, and the PWM signals are thus power-amplified. The power-amplified positive and negative PWM signals are converted into analog-waveform audio signals through the LPFs 102a and 102b, and the audio signals are provided to the speaker 103.
In the class-D operation of the above digital amplifying apparatus, a noise (hereinafter, “switching noise”) is generated during the switching by the positive and the negative PWM signals. The switching noise includes a noise component that can not be removed by the LPFs 102a and 102b. 
For example, when a switching frequency and the high frequency wave thereof, in the positive-phase amplifier 101a and the inverting amplifier 101b, are radio frequencies (or a frequency in the vicinity), a noise is generated in a reproduced signal, such as with radio reception of an audio signal. In this case, setting the cutoff frequency of the LPFs 102a and 102b at a lower frequency has a limit. Therefore, the noise component in the radio frequency band included in the switching noise is difficult to remove by the LPFs 102a and 102b. Hence, in this case, the remaining noise component that is not removed by the LPFs 102a and 102b interferes with the reception of broadcasts in the radio frequency band, and adversely affects the radio reception.
A method of removing such a remaining noise component can be, for example, a method of disposing a filter component between the LPFs 102a and 102b and the speaker 103 to remove the remaining noise component (see, for example, Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2003-046345